(1) Field of the Invention
This invention relates to a method and program for calculating ion distribution and, more particularly, to a method for calculating ion distribution in a crystalline member in the case of implanting ions in the member more than one time and a program for making a computer perform such a calculation.
(2) Description of the Related Art
Usually arsenic or boronic ions, for example, are implanted in a semiconductor substrate in the process of fabricating a semiconductor device to change the electrical properties of a member.
Ions may be implanted in extension and source or drain areas from four directions in the process of fabricating a metal oxide semiconductor field effect transistor (MOSFET).
FIG. 9 is a view for describing how to perform such ion implantation. As shown in FIG. 9, when ion implantation is performed in the process of fabricating a MOSFET, ions are implanted from four directions, that is to say, from the left-hand and right-hand directions (shown by arrows) and from the front and rear directions (not shown) to form n+ areas.
When such ion implantation is performed, the energy and dose of ions to be implanted must be determined so that desired ion distribution will be obtained. Conventionally, a personal computer or a workstation (hereinafter referred to simply as a computer) therefore has been used to simulate ion distribution.
By the way, the fact that if ions are implanted in a member having crystal structure, crystal structure in an area where the ions collide breaks down into amorphous structure is known.
It would be difficult for ions to go beyond an area where crystal structure has broken down. Accordingly, even if dose is increased, ion concentration in an area beyond an amorphous area will not increase.
FIG. 10 is a view for describing this phenomenon. Horizontal and vertical axes in FIG. 10 indicate the depth from the surface of a member where ions are implanted and the concentration of ions implanted, respectively. Each curve indicates results obtained when dose is changed.
As shown in FIG. 10, an increase in concentration at the peak portions (each corresponding to an amorphous area) of graphs is almost proportional to an increase in dose. On the other hand, concentration in an area (channeling area) beyond the amorphous area is almost constant regardless of an increase in dose.
Conventionally, to simulate a case where ion implantation is performed more than one time, a result obtained by performing ion implantation once has simply been added accumulatively by the number of times ion implantation is performed.
As stated above, the concentration of ions in an amorphous area shows linearity. Therefore, the principle of superposition applies and an obtained result approximates to the correct value.
However, the concentration of ions in a channeling area does not show linearity. Therefore, the principle of superposition does not apply and a result obtained by simply adding differs significantly from the correct value.